COMPRESSED AIR SYSTEMS A-Z COMP AIR COMPRESSOR TRAINIBG

Transcription

1 COMPRESSED AIR SYSTEMS 1

2 SCOPE OF WORK Various types of compressors and their characteristics Measurement of power consumption, free air delivery, operating pressure, Isothermal power required, and volumetric efficiency. Measurement of inlet air temperature Effect of inlet air temperature on power consumption Assessment of specific power consumptions 2

3 Effect of following on energy consumption: Dust in inlet air Dryness of inlet air Inter and after cooler temperature and pressure. Delivery pressure setting Size of pipe/filter choking, pressure drop, piping layout. Bends and fitting losses effect of fittings of different types and sizes. 3

4 Variable speed drive (VFD) for compressor operation and its assessment Compressor capacity assessment Capacity control methods Method of system leak quantification Detailed energy audit of a compressed air installation with actual case study 4

5 1. INTRODUCTION 1.1 Energy sources for operating equipments 1.2 Why compressed air? (Properties that make it important) 1.3 Compressors and their working principles 1.4 Highlights of compressors, industrial position 5

6 1.1 Energy sources for operating industrial equipments: In today s world, many methods are employed to operate industrial tools and equipments, to carry out the process work that these industries perform. Be it manufacturing industries, power plants, or process industries, there are always energy sources that act on the equipments, actuating them and making them function. 6

7 The commonly used sources are: Direct electricity (direct motor coupling) Steam Water Industrial oils Compressed air 7

8 1.2 Why compressed air? One of the major methods of actuation of industrial machines is pneumatic actuation, ie, action by compressed air. It is nothing but air, at a pressure higher than the atmospheric pressure. It has been widely used as an energy source for operating simple to sophisticated equipments in mechanical, chemical, and other process industries. 8

9 Even though air is the most abundant natural resource on the planet, compressed air is one the most expensive of services to provide. The following properties of compressed air justify why it s such an important energy source: Elastic nature (which permits internal storage of energy) Non toxicity Low viscosity (compared to other working fluids) No appreciable corrosive nature (increased life of equipments) 9

10 1.3 Compressors and their working principle An air compressor is basically a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air. This compressed air can be stored, or can be released in quick bursts, on command. Compressors work on two parallel principles: 10

11 Compressed air energy storage (CAES): Ideal gas law states that, for any state of an ideal gas, PV = mrt Where, P = absolute pressure of gas V = volume occupied by the gas T = absolute temperature of gas m = amount of substance present in the gas R = universal gas constant Therefore, to increase pressure of a gas, one has to either decrease the volume it occupies, or increase its temperature, or both. Invariably, compression of air creates heat, and the air is warmer after compression. 11

12 There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermal. o o o Adiabatic storage continues to keep the heat produced by compression and returns it to the air when the air is expanded to generate power. Diabatic storage dissipates much of the heat of compression with intercoolers (thus approaching isothermal compression) into the atmosphere as waste; essentially wasting, thereby, the renewable energy used to perform the work of compression. Isothermal compression and expansion approaches attempt to maintain operating temperature by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. In industry, the positive-displacement air compressors work on the diabatic storage of energy, and work by forcing air into a chamber whose volume is subsequently decreased, to compress the air. They are classified as reciprocating and rotary, depending on how the air is 12 guided into the chamber.

13 Bernoulli s equation: From Bernoulli s equation, for continuous flow of a fluid along a horizontal pipe, from section1 to section2, P 1 P 2 = (V 22 -V 12 )/2 Therefore, V 1 <V 2 means P 1 >P 2, and vice versa Thus, pressure of a gas in motion increases at the cost of its velocity, when it flows across a duct. This principle is employed in dynamic compressors, which use centrifugal force generated by spinning impellers to accelerate and then decelerate captured air, which pressurizes it. 13

14 1.4 Overview and industrial position of compressors In industry, compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water. About 8-10% of all electrical power used in an industry is employed in the process of compressing air. 14

15 Air compressors are used in a variety of industries for the following purposes: o Operating machinery like hammers, presses o Supplying process requirements o Supplying air at high pressure for chemical action 15

16 Along with industrial pneumatic work, air compressors are also used to produce compressed air in the following processes o Vehicle propulsion o Energy storage o Scuba diving o Air brakes o Ammunition propulsion 16

17 Air compressors are machines with very low overall efficiency; only 10-30% of the energy supplied to the compressors reaches the point of end use. The remaining power is converted to unusable heat energy, and to a smaller extent, is lost in friction, misuse, and noise. These factors need to be identified and minimized for 17 optimum performance.

18 2. TYPES OF COMPRESSORS 2.1 Classification basis Positive displacement compressors o Reciprocating compressors o Rotary compressors Dynamic compressors o Centrifugal compressors o Axial compressors 2.2 Selection criteria for compressors 2.3 Comparison of compressors 18

19 2.1 Types of compressors Depending on the action performed on air, compressors are broadly classified as follows: Compressor types Positive displacement Dynamic Reciprocating Rotary Centrifugal Axial 19

20 2.1.1 Positive displacement compressors In these compressors, pressure of the air is increased by reducing its volume, in accordance with the ideal gas law. These are further divided into reciprocating and rotary compressors. 20

21 Reciprocating compressors Reciprocating compressors are the most widely used type of compressors. In these compressors, air compressed by the action of a piston in a cylinder with one-way valves and pumped into an air chamber (see figure). 21

22 Subtypes of Reciprocating Compressor Depending on the machine configuration and geometry, these compressors are available as horizontal, vertical, horizontal opposed-balanced and tandem compressors Horizontal opposed balanced compressor 22

23 Depending on the capacity, these compressors can have single or multiple cylinders, and can be single-acting or double-acting. These compressors can be lubricated with oil or nonlubricated. Non lubricated machines have higher specific power consumption (kw/cfm) as compared to lubricated types. 23

24 Depending on the number of compression stages, there are single stage or multi stage compressors. In multi stage compressors, air is cooled in between a stage of compression with the help of an intercooler. Two-cylinder, two-stage reciprocating compressor 24

25 Multi stage compressors have the following features over single stage compressors: Higher discharge temperature of air Lower specific power consumption for same pressure differential Higher investment costs for applications with high discharge pressure (>7 bar) and low capacities (<25cfm). Reduced pressure differential across cylinders, reducing load on the compressor components, thereby increasing life of the machine. 25

26 Depending on the method of cooling, they are classified as air cooled and water cooled. o Single cylinder compressors are generally air cooled, while multi cylinder compressors are generally water cooled. o Water cooled systems are more energy efficient than air cooled types. 26

27 Variants of reciprocating compressors: A Diaphragm compressor is a variant of a reciprocating compressor which uses a flexible membrane, usually made of rubber or silicone, instead of a metal piston, to compress the air. Due to limited stiffness of these materials, these are used in low pressure applications, like compression of hydrogen or CNG. 27

28 An Ionic liquid piston pump is a highly efficient hydrogen compressor which employs a liquid piston instead of a metal piston, to compress hydrogen up to 450 bar. o o o It works on the principle of poor solubility of hydrogen in an ionic liquid. The heat exchangers that are used in a normal piston compressor are removed, as the heat is removed in the cylinder itself where it is generated. Almost 100% of the energy going into the process is utilized with little energy wasted as reject heat. 28

29 2.1.2 Rotary compressors These compressors use rotors in place of pistons, giving a pulsating free discharge air. These rotors are power driven. They have the following advantages over reciprocating compressors: o They require a lower starting torque o They give a continuous, pulsation free discharge air o They generally provide higher output o They require smaller foundations, vibrate less, and have lesser parts, which means less failure rate 29

30 Subtypes: Depending on the action and geometry of the rotors, these are classified as follows: Rotary compressors Lobe compressors Screw compressors Vane compressors Scroll compressors 30

31 Lobe compressor Also called the roots blower, it s essentially a low pressure blower and is limited to a discharge pressure of 1 bar in a single-stage and 2.2 bar in two stage-design. 31

32 Rotary screw compressors: Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the air into a smaller space (see figure). These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Because of simple design and few wearing parts, rotary screw air compressors are easy to install, operate, and A-Z maintain. COMP AIR COMPRESSOR TRAINIBG 32

33 Rotary screw compressors are commercially produced in Oil flooded and Oil free types: Oil flooded compressors are nothing but oil cooled compressors; where oil seals the internal clearances of the compressor. Though filters are needed to separate the oil from the discharge air, cooling takes place right inside the compressor, and thus the working parts never experience extreme operating temperatures leading to prolonged life. The oil free screw air compressors use specially designed air ends to compress air, giving true oil free air. They are water cooled or air cooled and provide the same flexibility as oil A-Z COMP flooded AIR COMPRESSOR rotary TRAINIBG compressors 33

34 Rotary vane compressors: One of the oldest compressor technologies, rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. 34

35 The rotor is mounted offset in a larger housing that is either circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades They can be either stationary or portable, can be single or multistaged, and can be driven by electric motors or internal combustion engines. They are well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor. They can have mechanical efficiencies of about 90% 35

36 Scroll compressors: A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress air. Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls (see figure). 36

37 Due to minimum clearance volume between the fixed scroll and the orbiting scroll, these compressors have a very high volumetric efficiency. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range. 37

38 2.1.2 Dynamic compressors In these compressors, velocity of air is increased, which is converted to high pressure at the outlet of the compressor by varying the area of flow. In these compressors, a small change in compression ratio produces a marked change in compressor output and efficiency. They are better suited for applications involving very high capacities (> 12,000 cfm). Dynamic compressors are classified centrifugal compressors and axial compressors. 38

39 Centrifugal air compressors: Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section, also called plenum, converts the velocity energy to pressure energy, hence we get air at high pressure (see figure). 39

40 They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. They are generally single stage machines, but with multiple staging, they can achieve high output pressures (>69 MPa). The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents. It s a continuous output machine. 40

41 Axial compressors: Axial flow compressors are dynamic rotating compressors that use arrays of fan-like airfoils to progressively accelerate air. The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary. The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. (See figure) 41

42 Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial mach number They are used where there is a requirement for a high compression ratio, high flow rate or a compact design. 42

43 2.2 How to select an appropriate compressor? When choosing a compressor, the parameters of suitability are generally the range of capacity of free air delivery, and the working pressure. The general selection criterion for compressors is given by the National Bureau of Energy Efficiency, as follows: 43

44 Capacity and pressure ranges for types of compressors 44

45 Working range of various compressors 45

46 2.3 Comparison on basis of unloading/part-load operation power consumption Along with working pressure, delivery capacity and efficiency, the variations in power consumptions during unloading/ part-load operation are crucial in cost estimations, and depend on the type of compressor, and method of capacity control employed, the term capacity control to be discussed later. The relative efficiencies of compressors at various loading conditions are as follows: 46

47 Relative efficiency of compressors at various loading conditions 47

48 It can be seen that reciprocating compressors are most efficient at part load conditions as well as no load conditions. Screw compressors, on the other hand, perform poorly no load conditions. The unload power consumption is marginally higher than that of reciprocating compressors. That is why some of the plants operate screw compressors at full load for meeting the base-load requirement, and reciprocating compressors for fluctuating load to optimize on unload power consumption. However, for constant airflow requirement, it s preferable to install screw compressors. In case of fluctuating loads, it is desirable to have a screw compressor with variable speed drive to further optimize unload power consumption. 48

49 3. COMPRESSED AIR SYSTEM COMPONENTS Intake air filters Compressor Inter-stage & after coolers Air dryers Moisture drain taps Receivers Distribution through piping 49

50 The following are the components of a compressed air system which are crucial to the working of the system. The compressor takes in air and compresses it to the required pressure good control and maintenance is the key to saving energy here. 50

51 The air receiver is a pressure vessel that acts as a reservoir to store and cool the compressed air and helps to ensure reduced pressure variations from the compressor. The air receiver serves the following functions: o It dampens pulsations entering the discharge line from the compressor o Serves as a reservoir for sudden or unusually heavy demands in excess of compressor capacity o Prevents too frequent loading and unloading (short cycling) of the compressor o separates moisture and oil vapour, allowing the moisture carried over from the after coolers to precipitate 51

52 Inter stage coolers cool the air before it enters the next stage inside the compressor, to reduce the work of compression and increase efficiency. Section view of a water cooled intercooler 52

53 After coolers remove moisture in the air by reducing temperature in a water-cooled heat exchanger. Air cooler Water cooler 53

54 Intake air filters and air dryers treat the air to remove impurities such as water, dirt and oil that are present in the ambient air being drawn into the compressor and those added by the compressor. Filters cater to dust impurities, while driers remove moist impurities. Various types of dryers are used, like adsorbent dryers, refrigerant dryers, heat of compression dryers. 54

55 The impurities are filtered out, with liquids removed from the system by condensate traps. These traps resemble steam traps and can be manually operated (manual drain cocks) or timed/automated (automatic drain valves). The systems normally consists of the above components feeding a distribution system running throughout the factory to the end-use equipment, such as pneumatic drills supplied through a flexible hose. The figure shows how these components can be set up in a system. The arrangement can be more compact in case of small scale production. 55

56 A typical compressed air system with components and network 56

57 4. ENERGY PERFORMANCE ASSESSMENT OF COMPRESSORS 4.1 Why performance assessment? 4.2 Performance terms and definitions 57

58 4.1 Why performance assessment? Over a period of time, both performance of compressors and compressed air systems reduces drastically. This is mainly attributed to poor maintenance, wear & tear, etc. These lead to additional compressor installations which further reduce the efficiency. Therefore, a periodic performance assessment is essential to minimize the cost of compressed air. 58

59 4.2 Performance terms & definitions Compression ratio (r): It s the ratio of absolute discharge pressure at last stage, to the absolute intake pressure Compression ratio r = Where, P 1 = absolute intake pressure P 2 = absolute delivery pressure 59

60 Capacity: The capacity of a compressor is the full rated volume of flow of gas compressed and delivered under standard conditions of total temperature, total pressure, and composition prevailing at the compressor inlet. 60

61 Free air delivery (Q F ): It s the actual flow rate, rather than rated volume of flow. It s called free air delivery (FAD) because it means air flow rate at atmospheric conditions at any specific location, and not standard conditions. There are many methods to calculate FAD, as discussed in unit 5. 61

62 Compressor load: The loads on any air compressor are system frictional resistance, piping backpressure and the head the load imposes. Input power: It s the shaft horsepower supplied to the compressor including mechanical and electrical losses in the drive system. This is the power that determines the electric bill. 62

63 Isothermal power: It s the least power required to compress air assuming isothermal (constant temperature) compression conditions. Isothermal power (kw) = Where, P 1 =Absolute inlet pressure (kg/cm 2 ) r = compression ratio Q f = Free air delivered (m 3 /hr) 63

64 Isothermal efficiency: It s the percentage ratio of isothermal power to shaft power supplied. Isothermal efficiency = 64

65 Volumetric efficiency: It s the ratio of free air delivered to the compressor swept volume. Volumetric efficiency = Compressor displacement = Where, Q f = Actual free air delivery of compressor D = cylinder bore, metre L = cylinder stroke, metre S = compressor RPM x = 1 for single acting, and 2 for double acting cylinders n = number of cylinders 65

66 Specific power consumption: It s defined as input power (kw) per unit volume flow rate (m 3 /h). Specific power consumption = kwh/m 3 66

67 5. CAPACITY ASSESSMENT OF COMPRESSORS 5.1 Why capacity assessment? 5.2 Nozzle method 5.3 Pump up method 67

68 5.1 Why capacity assessment? The compressed air system is not only an energy intensive utility but also one of the least energy efficient, which calls for periodic performance assessment. One of the ways of assessing performance of the compressor is by measuring capacity of the compressor. A Cubic feet per minute (cfm) is the standard unit of measuring air flow rate. (1cfm = m 3 /h = m 3 /min = m 3 /s) There are many methods to assess compressors on their capacity basis, two of them being discussed here. 68

69 5.2 Nozzle method Principle: If a specially shaped nozzle discharges air to the atmosphere from a receiver, getting its supply from a compressor, sonic flow conditions set in at the nozzle throat, for a particular ratio of upstream pressure (receiver) to the downstream pressure (atmospheric) i.e. Mach number equals one. When the pressure in the receiver is kept constant for a reasonable interval of time, the airflow output of the compressor is equal to that of the nozzle and can be calculated from the known characteristic of the nozzle. 69

70 Measurements and working: o The compressor is started with the air from the receiver discharging to the atmosphere through the throttle valve and the flow nozzle. (See figure) 70

71 o It is ensured that the pressure drop through the throttle valve should be equal to or twice the pressure beyond the throttle. o After the system is stabilized the following measurements are carried out: o Receiver pressure (P 2 ) o Pressure and temperature before the nozzle (P 3, T3) o Pressure drop across the nozzle (P 3 -P 4 ) o Speed of the compressor (RPM) o kw, kwh and amps drawn by the compressor 71

72 Measuring instruments required for test: Manometer Standard nozzle Thermocouple Differential manometer Psychrometer Tachometer Electric demand analyzer 72

73 Nozzle sizes: The following sizes of nozzles are recommended for the range of capacities indicated below: Flow nozzle with profile as desired in IS 10431:1994 and dimensions 73

74 Calculations required: Free air delivered, Q F (m 3 /sec) = Where, k: Flow coefficient as per IS d: Nozzle diameter (metre) T1: Absolute inlet temperature ( K) P1: Absolute inlet pressure (kg/cm 2 ) P3: Absolute Pressure before nozzle (kg/cm 2 ) T3: Absolute temperature before nozzle ( K) Ra: Gas constant for air (287.1 J/kg k) P3 P4: Differential pressure across the nozzle (kg/cm 2 ) 74

75 Worked example 1 Calculation of Isothermal Efficiency for a Reciprocating Air Compressor with the following data: k: Flow coefficient (Assumed as 1) d: Nozzle diameter : 0.08 metre P2: Receiver Pressure : 3.5 kg / cm2 (a) P1: Inlet Pressure : 1.04 kg / cm2(a) T1: Inlet air temperature : 30 C or 303 K P3: Pressure before nozzle : 1.08 kg / cm2 T3: Temperature before the nozzle : 40 C or 313 K P3 P4: Pressure drop across the nozzle : kg / cm2 Ra: Gas constant : 287 Joules / kg K 75

76 Step 1: Calculate Volumetric Flow Rate Free air delivered Q F = = = m 3 /sec = m 3 /h 76

77 Step 2: Calculate Isothermal Power Requirement Isothermal Power (kw) = P1 - Absolute intake pressure = 1.04 kg / cm 2 Qf - Free Air Delivered = m3 / h. Compression ratio r = 3.51/1.04 = 3.36 Isothermal Power (kw) = = kw 77

78 Step 3 : Calculate Isothermal Efficiency Motor input power = 100 kw Motor and drive efficiency = 86 % Compressor input power = 86 kw Isothermal efficiency = = = 56% 78

79 5.3 Pump up method: Also known as receiver filling method, this method can be adopted where the elaborate nozzle method is difficult to be deployed. It s a less accurate but simple method, and is employed at a smaller scale, to measure capacity of a compressor in a shop floor. Principle: The time taken by the air to fill an empty receiver of known volume is inversely proportional to the flow rate of the discharge air. This principle is used in this method. 79

80 Procedure: Isolate the compressor along with its individual receiver being taken for test from main compressed air system by tightly closing the isolation valve or blanking it, thus closing the receiver outlet. Open water drain valve and drain out water fully and empty the receiver and the pipeline. This process is called bleeding. Make sure that water trap line is tightly closed once again to start the test. Start the compressor and activate the stopwatch. Note the time taken to attain the normal operational pressure P2 (in the receiver) from initial pressure P1. Calculate the capacity as per the formulae given below: 80

81 Calculations required Actual free air discharge Q F = Where P 2 = Final pressure after filling (kg/cm 2 ) P 1 = Initial pressure (kg/cm 2 ) after bleeding P 0 = Atmospheric Pressure (kg/cm 2 ) V = Storage volume (in m 3 ) which includes receiver, after cooler, and delivery piping T = Time take to build up pressure to P 2 in minutes m 3 /minute Note: The above formula holds good only for isothermal compression of air; in case the actual compressed air temperature at discharge, say t 2 C is higher than ambient air temperature say t 1 C (as is usual case), the FAD is to be corrected by a multiplication factor 81

82 Worked example 2 An instrument air compressor capacity test gave the following results (assume the final compressed air temperature is same as the ambient temperature) - find the Free air discharge of the compressor. Receiver Volume : 7.79 m 3 Additional hold up volume, i.e., pipe / water cooler, etc : m 3 Total volume : m 3 Initial pressure P1 : 0.5 kg/cm 2 Final pressure P2 :7.03 kg/cm 2 Atmospheric pressure P0 :1.026 kg/cm 2 Time taken to buildup pressure from P1 to P2 : min 82

83 Free air discharge (m3/minute) = = = m 3 /minute 83

84 6. ASSESSMENT OF SPECIFIC POWER CONSUMPTION 6.1 Why specific power consumption assessment? 6.2 Calculations 84

85 6.1 Why specific power consumption assessment? Practically, specific power consumption of a compressor is the most effective guide in comparing compressor efficiencies. This makes its assessment important from an energy conservation point-of-view. 85

86 Calculations: As discussed earlier, specific power consumption is defined as input power (kw) per unit volume flow rate (m 3 /h), i.e., Specific power consumption = Therefore, knowing the input/shaft power supplied to the compressor, and the actual free air discharge, specific power consumption of the compressor can be calculated. 86

87 Worked example 3 Calculate the specific power consumption in the compressor described in Example 1. In Example 1, The measured flow rate : m 3 /hr and actual power consumption : 100 kw Therefore, the specific power requirement = 100/ = kw-h/m 3 87

88 7. FACTORS AFFECTING COMPRESSOR PERFORMANCE 7.1 Inlet air temperature 7.2 Dust inlet in air 7.3 Dryness of inlet air 7.4 Altitude/elevation 7.5 Performance of inter-stage & after coolers 7.6 Delivery pressure setting 7.7 Pressure drop in flow through pipes 7.8 Bends & fitting losses 88

89 7.1 Inlet air temperature As a thumb rule, Every 4 0 C rise in inlet air temperature results in a higher energy consumption by 1 % to achieve equivalent output. Hence, cool air intake reduces work of compression leads to a more efficient compression. Effect of Inlet temperature on power consumption 89

90 It is preferable to draw cool ambient air from outside, as the temperature of air inside the compressor room will be a few degrees higher than the ambient temperature. Precaution: While extending air intake to the outside of building, care should be taken to minimize excess pressure drop in the suction line, by selecting a bigger diameter duct with minimum number of bends. 90

91 7.2 Dust in inlet air Dust in the suction air causes excessive wear of moving parts and results in malfunctioning of the valves, due to abrasion. Suitable air filters should be provided at the suction side, with high dust separation capacity, low-pressure drops and robust design to avoid frequent cleaning and replacement. 91

92 Effect of Pressure Drop across Air Inlet Filter on Power Consumption Hence, it is advisable to clean inlet air filters at regular intervals to minimize pressure drops. Manometers or differential pressure gauges across filters may be provided for monitoring pressure drops so as to plan filtercleaning AIR COMPRESSOR schedules. A-Z COMP TRAINIBG 92

93 7.3 Dryness of inlet air Atmospheric air always contains some amount of water vapour, depending on the relative humidity, being high in wet weather. The moisture level will also be high if air is drawn from a damp area - for example locating compressor close to cooling tower, or dryer exhaust. Moisture in Ambient Air at Various Humidity Levels The moisture-carrying capacity of air increases with a rise in temperature and decreases with increase in pressure. 93

94 Dewpoint and Condensation- When air with a given relative humidity is cooled, it reaches a temperature at which it is saturated, i.e., the air contains as much water vapour as it can hold. The temperature at which the air is at 100% relative humidity is known as the dewpoint of the air. Moisture levels at given dew point 94

95 Cooling air beyond the dewpoint results in condensation of the water vapour. This can be anything, from a nuisance to a serious problem, depending on the application. Typically it causes cylinder damage. For safety purposes, dewpoint should be kept as minimum as possible, using air dryers. 95

96 7.4 Elevation The altitude of a place has a direct impact on the volumetric efficiency of the compressor. As the compression ratio is higher at higher altitudes, compressors located at these altitudes (example turbochargers in aircraft engines) consume more power to achieve a particular delivery pressure than those at sea level. Effect of Altitude on Volumetric Efficiency 96

97 7.5 Performance of Inter and After Coolers As discussed, inter-coolers are provided between successive stages of a multi-stage compressor to reduce the work of compression (power requirements) - by reducing the specific volume through cooling the air - apart from moisture separation. Ideally, the temperature of the inlet air at each stage of a multistage machine should be the same as it was at the first stage. This is referred to as perfect cooling or isothermal compression. But in actual practice, the inlet air temperatures at subsequent stages are higher than the normal levels, resulting in higher power consumption, as a larger volume is handled for the same duty. 97

98 Effect of Inter-stage Cooling on Specific Power Consumption of a Reciprocating Compressor It can be seen that an increase of C in the inlet air temperature to the second stage results in a 2 % increase in the specific energy consumption. 98

99 Use of water at lower temperature reduces specific power consumption. However, very low cooling water temperature could result in condensation of moisture in the air, which if not removed would lead to cylinder damage. Similarly, inadequate cooling in after-coolers (due to fouling, scaling etc.), allow warm, humid air into the receiver, which causes more condensation in air receivers and distribution lines, which in consequence, leads to increased corrosion, pressure drops and leakages in piping and end-use equipment. Periodic cleaning and ensuring adequate flow at proper temperature of both inter coolers and after coolers are therefore necessary for sustaining desired performance. 99

100 7.6 Pressure Settings Compressors operate between pressure ranges called loading (cut-in) and unloading (cut-out) pressures. Loading and unloading is done using a pressure switch. The pressure switches must be adjusted such that the compressor cuts-in and cuts-out at optimum levels. They should not be operated above their optimum operating pressures as this not only wastes energy, but also leads to excessive wear, leading to further energy wastage. For the same capacity, a compressor consumes more power at higher pressures. The volumetric efficiency of a compressor is also less at higher delivery pressures. 100

101 The possibility of lowering (optimising) the delivery pressure settings should be explored by careful study of pressure requirements of various equipments, and the pressure drop in the line between the compressed air generation and utilization points. A reduction in the delivery pressure by 1 bar in a compressor would reduce the power consumption by 6 10 %. Typical Power Savings through Pressure Reduction 101

102 7.7 Minimum pressure drop in air lines Excess pressure drop due to inadequate pipe sizing, choked filter elements, improperly sized couplings and hoses lead to more power consumption, hence energy wastage. Typical acceptable pressure drop in industrial practice is 0.3 bar in mains header, at the farthest point, and 0.5 bar in distribution system. Typical Energy Wastage due to Smaller Pipe Diameter for 170 m 3 /h(100 cfm) 102

103 7.8 Losses in bends and fittings Not only piping, but fittings and bends in those pipes are also a source of pressure loss. Following are some common fittings used in compressed air system. Gate valve Tee 90 long bend Elbow 103 Return bend Tee globe valve

104 Resistance of Pipe Fittings in Equivalent Lengths (in metres) 104

105 7.9 Surges in centrifugal compressors What are surges? o o o o Surge is defined as the operating point at which centrifugal compressor peak head capability and minimum flow limits are reached. When the plenum pressure behind the compressor is higher than the compressor outlet pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the plenum pressure will decrease, inlet pressure will increase and the flow reverses again. This phenomenon, called surge, repeats and occurs in cycles with frequencies varying from 1 to 2 Hz. 105

106 Effects on performance: o o Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor. Surge points for centrifugal compressors running at varying speeds 106

107 Anti surge control systems o These systems detect when a process compression stage is approaching to surge and subsequently take action to reverse the movement of the operating point towards the surge line (SL). This decreases the plenum pressure and increases the flow through the compressor, resulting in stable working conditions. Shifting of operating point away from surge point It is normally achieved by opening an Anti-Surge Control Valve (or ASCV), returning the discharge gas to the inlet of the compressor via a suction cooler. resulting increase in compressor inlet volume flow moves the operating point away from surge. 107

108 8. QUANTIFICATION OF AIR LEAKAGE 8.1 Causes of air leakage 8.2 Detection 8.3 Method of leak quantification 108

109 8.1 Sources of air leakage Leaks not only waste energy, but also reduce the effective capacity of compressor plant and may in extreme cases, reduce the performance of the equipment significantly. Leaks should be always rectified before any increase in compressor capacity is planned. The sources of leakage are numerous, but the most frequent causes are: o o o o o o o o Manual condensate drain valves left open Failed auto drain valves Shut-off valves left open Leaking hoses and couplings Leaking pipes, flanges and pipe joints Strained flexible hoses Leaking pressure regulators Air-using equipment left in operation when not needed 109

110 The amount of free air wasted also depends on size of the orifice through which air escapes. Discharge of air (m 3 /min) through orifice of given diameter (orifice constant C d =1) 110

111 8.2 Detection While large leakages can be detected easily using the hissing sound produced, leakage tests for small leakages are conducted by soap application or ultrasonic vibration Ultrasonic detection: o o o Principle- Leak testing is done by observing and locating sources of ultrasonic vibrations created by turbulent flow of gases. This method uses a leak detector having a sensing probe, which senses when there are leakages in compressed air systems at high temperatures, i.e., beneath insulated coverings, pipelines etc. See how leakage is detected: 111

112 8.3 Method of leak quantification The following procedure is adopted for simple shopfloor leak quantification: Shut off compressed air operated equipments. Run the compressor to charge the system to set operating pressure, so that the compressor unloads. Note the time. Due to air leakage the system pressure will fall, and the compressor will load again. Note the time. Note the period for which the compressor is on load and off load at least 8-10 times for 112 accurate mean values of each.

113 Calculations involved The system leakage is calculated as : %leakage = System leakage quantity, q (m 3 /min) = Where, Q = compressor capacity (m 3 /min) T = Time on load in minutes t = Time on unload in minutes 113

114 Worked example 4 In the leakage test in a process industry, following results were observed: Compressor capacity (m 3 /minute) = 35 Cut in pressure (kg/cm 2 ) = 6.8 Cut out pressure (kg/cm 2 ) = 7.5 Load power drawn (kw) = 188 Unload power drawn (kw) = 54 Average load time T (minutes) = 1.5 Average unload time t (minutes) = 10.5 Comment on leakage quantity and avoidable loss of power due to air leakages. 114

115 Leakage quantity, q (m 3 /min) = = m 3 /min Leakage quantity per day (m 3 /day) = (4.375) x (24) x (60) = 6300 m 3 /day 115

116 Specific power for compressed air generation = = kwh/m 3 Energy lost due to leakage/day = (0.0895) x (6300) = 564 kwh 116

117 9. CAPACITY CONTROL METHODS 9.1 Need for capacity controlling 9.2 Various methods o Automatic on-off control o Load-unload control o Multi-step control o Throttling control o Variable speed drive control 117

118 9.1 What is capacity control? In many installations, the use of air is intermittent. This makes it necessary to control the output flow of the compressor. This process is called is called capacity control of compressors. The type of capacity control chosen has a direct effect on the compressor power consumption. Some commonly used methods are discussed as follows 118

119 9.2.1 Automatic ON/OFF control: In this method the compressor is started or stopped by means of a pressure activated switch as the air demand varies. This is a very efficient method of capacity control, where the motor idle running losses are eliminated, as it completely switches off the motor when the set pressure is reached. This method is suitable for small compressors. 119

120 9.2.2 Load and unload control This is a two step control method where compressor is loaded when there s air demand and unloaded when there is no air demand. During unloading, a positive displacement compressor may consume up to 30% of the full load power, depending upon the type, configuration operation and maintenance practices. 120

121 9.2.3 Multi-step control Large capacity reciprocating compressors are usually equipped with a multi step control. In this type of control, unloading is accomplished in a series of steps (0%, 25%, 50%, 75% and 100%) varying from full load down to no load. Power consumption of a typical reciprocating compressor at various loads 121

122 9.2.4 Throttling control The usual way of regulating compressor capacity relies on controlling inlet or exhaust valves, to restrict the output of the compressor while it continues to run at full speed. This is called throttling control. Throttling control in centrifugal compressors: The capacity of centrifugal compressors can be controlled by throttling using inlet guide vanes and butterfly valves. In throttling terminology, Turndown is the ratio of maximum to minimum flow. For a turndown of 10:1, if you have a maximum flow of 5,000 SCFM, you can expect to maintain stable control A-Z COMP down AIR COMPRESSOR to 500 TRAINIBG SCFM. 122

123 Butterfly valve: It s a valve mounted on or near the first stage inlet, and it closes in reaction to a rise in system pressure. As the pressure drop across the valve increases, the density of the entering air decreases. This results in a lower mass flow in relation to inlet ambient cfm. Power draw falls, but not proportionally to the decrease in mass flow. This implies the specific power consumption (kw/cfm) falls. Additionally, as the butterfly valve reaches the end of its closure, it produces turbulence, which further reduces the effective flow into the impeller. Since the pressure always decreases across the butterfly valve, it can do nothing to improve compression efficiency or specific power. 123

124 Inlet guide vanes: Inlet guide vanes are usually mounted on the compressor's first stage inlet, but are often installed on each stage in larger process units. Like butterfly valves, inlet guide vanes vary the volumetric flow at constant discharge. Inlet guide vanes produce a swirl in the airflow, usually in the direction of impeller rotation. When throttling flow, the vanes shift from being parallel to the air stream to fully perpendicular. The net result is reduced input power and improved specific power at low flow. Inlet guide vanes reduce the power required to produce a lower-than-design flow at the same pressure more than an inlet butterfly valve can reduce it. Hence, inlet guide vanes are more efficient at turndown control than butterfly valves. 124

125 9.2.5 Speed control Another efficient way to match compressor output to meet varying load requirements is by speed control. In this method, motor running speed is changed by manually varying electrical supply to the motor. Typical part load gas compression: power input for speed and vane control of centrifugal compressors At low volumetric flow(less than 40%); Vane control may result in lower power input due to low efficiency of the speed control system. For loads more than 40%, speed control is recommended. 125

126 10. VARIABLE SPEED DRIVE (VSD) 10.1 What is variable speed drive? 10.2 How to incorporate VSD? 10.3 Checklist for retrosetting VSD 10.4 Advantages and limitations of VSD 126

127 10.1 What is variable speed drive? A variable-speed drive (VSD) air compressor is an air compressor that uses variable-speed drive technology for capacity control. This type of compressor uses a special drive to control the speed (RPM) of the motor, and hence the compressor pulley, which in turn saves energy compared to a fixed speed equivalent. In this way, when demand for air reduces, so does power consumption. 127

128 Variable frequency drive The most common form of VSD technology in the air compressor industry is a variable-frequency drive, which converts the incoming AC power to DC and then back to a quasi-sinusoidal AC power using an inverter switching circuit. Variation of torque and power with frequency of power supplied to the motor This way it adjusts the frequency of the power supplied to the motor, thereby changing the induction or synchronous motor speed. 128

129 10.2 How to incorporate VSD? There are two ways to incorporate VSD in a compressor, o An existing compressor of a suitable size can be retrofitted o A new compressor with built in VSD can be installed Compressor with retrofit VSD Because the air demands may vary throughout the day, we need to check at a range of times to estimate the average load. Once the average load of the compressor is known, an assessment can be made of the likely savings and how VSD can be incorporated in the compressed air system. Variable speed technology can be applied to most air compressors and they are available as new machines from 5kW to over kw.

130 Cost comparison of retrofits and new compressors with inbuilt VSDs New compressors with built in VSD operate over a wide speed range and often have special features that would not be part of a retrofit, like direct coupled motors eliminating gearbox or belt drive losses. Purpose built VSD compressors will normally have a wider control band than retrofitted units giving better efficiency across a range of demands, and will also be backed by A-Z the COMP AIR manufacturer s COMPRESSOR TRAINIBG warranty. 130

131 10.3 Checklist for retrofitting VSD Retrofit of existing machines can give good benefits at lower cost than buying new ones, but great care must be taken when retrofitting VSDs on rotary screw and piston machines to ensure that o Correct levels of lubrication are maintained o Vibration problems are avoided o Cooling is not compromised. For example, in two stage oil free compressors, manufacturers generally advise against retrofit. 131

132 10.4 Advantages and limitations of VSD in compressors The advantages of variable speed drives are: o Efficiency over fixed speed machines is improved at part load conditions under 75%. o Pressure fluctuations are eliminated, which often reduces system generation pressure by up to 0.5 bar, giving extra energy savings. o Variable speed controls provide soft starting eliminating high inrush currents. Thus avoiding power surges. 132

133 There are certain limitations to the use of VSDs: o o o o o There is heavy expense associated with the drive, and the sensitivity of these drives specifically to heat and moisture. VSD retrofit should not be considered on centrifugal machines as they usually run above the first critical speed and serious damage can occur if the speed is changed. Variable speed drive compressors are not necessarily appropriate for all industrial applications. If a variable speed drive compressor operates continuously at full speed, the switching losses of the frequency converter result in a lower energy efficiency than an otherwise identically sized fixed speed compressor. Although VSDs do save money under most load conditions, the extra cost is only justified when the average load on the compressor is less than 75%. VSDs if not properly operated, can lead to harmonic distortion in the power supply, which increases supply costs at the transformer. 133

134 11. CONTROLLING MULTIPLE COMPRESSORS 11.1 Cascade control 11.2 Electronic sequential control 134

135 The trouble with multiple controlling is that even compressors with energy-efficient part-load controls can be inefficient when operated together. Many multiple compressor controllers available today use different logics to solve the same problem. o Some controllers merely start and stop compressors based on system pressure; some rely on time of day to determine which compressors to run. o Others will work only with specific compressor types or those from only one manufacturer. Two mainstream methods are adopted for capacity control of multiple compressors. 135

136 11.1 Cascade control This form of multiple compressor control has been in use for a long time. Multiple compressors are controlled based solely on system pressure. As system pressure falls below a set point, additional air compressors are brought online in a predetermined sequence. They are shut down when the desired pressure is reached. 136

137 This controller requires the plant to operate with a cascading pressure band (see figure). Traditional cascading set point control scheme However, it is not the most efficient method as it only runs the system at minimum pressure at the periods of highest demand. Cascade can also create a large pressure differential, usually 50kPa (0.5bar) per installed compressor. They are best suited for load-unload screw or reciprocating compressors. 137

138 11.2 Electronic sequential control A more efficient method of controlling multiple compressors is via an electronic sequential controller, which can control multiple compressors around a single set pressure, narrowing down the pressure band to a great extent. With programmable logic controllers, modern sequencers have evolved with the ability to determine the most efficient combination of available compressors to meet the ever-changing plant demand efficiently and effectively. Interface of electronic smart sequential controller 138

139 These systems make compressors available to match demand as closely as possible. They can also predict when to start/stop or load/unload the next compressor in sequence by monitoring the decay/rise in system pressure. So, rather than leaving potentially redundant compressors idling, the system shuts them down after a predetermined period. Cascade control vs. electronic control 139

140 Smart controllers have many additional practical capabilities, including the ability to display the plant s compressed air volumetric consumption rate; input power required to produce the compressed air; system efficiency data; peak, average and minimum demands throughout the day; and equipment status and alarms (see figure) Display screen of smart controller 140

141 12. ENERGY CONSERVATION OPTIONS FOR COMPRESSED AIR SYSTEM 12.1 Avoiding misuse of compressed air 12.2 Leak reduction 12.3 Capacity control 12.4 Compressor modulation by Optimum Pressure Settings 12.5 Improving intake air quality, generation and control 12.6 Air receivers 12.7 Piping layout 12.8 Waste heat recovery 12.9 Checklist for Energy Efficiency in Compressed Air System 141

142 A huge part of compressor expenses makes up for energy costs (almost 73%). This makes energy conservation options indispensable for an industry with a compressed air system. Compressor costs over a ten year life Following are the primary steps for energy saving in a compressed air system. 142